The present invention relates to a printing head for a wire dot printer, and more particularly to a low noise, high speed electrodynamic printing head for a wire dot printer.
As print-out devices for use with computers, wire dot printers of the type having a clapper type printing head are widely known. This clapper type printing head has a wire actuator with an armature attached to one end thereof. When the armature is attracted by an electric magnet, the wire actuator is given an impact at its top end so as to hit an ink ribbon from its back to print an ink dot on a printing paper.
Because the clapper type wire dot printer is very noisy due to the impact given by the armature, a low noise wire dot printer has been desired. A wire dot printer that is hopeful as a low noise type is of an electrodynamic type wire dot printer disclosed in, for example, Japanese patent unexamined publication No. 60--206,669. The electrodynamic type wire dot printer taught by the above Japanese patent unexamined publication comprises an actuator having a printing pin disposed in a magnetic field. This actuator comprises two curved wires which are connected to each other at one ends where the printing pin is fixed and, on the other hand, fixedly supported at the other ends with keeping a distance therebetween, so as to be formed in a generally Y-shaped configuration. By passing an electric current pulse through the actuator from one wire to the other wire to generate an electromagnetic force, the actuator is resiliently deformed to give the printing pin a thrust movement. Since, in the electrodynamic type printing head, the printing pin can hit the platen roller around which a printing paper is wrapped without being accompanied with impact, the printing head does not produce large noise and can be simple in structure.
There is known an improved electrodynamic type wire dot printing head, namely a transversal type wire dot printing head. The transversal type wire dot printing head has a straight, transversely disposed wire actuator with a printing pin held approximately at the middle. The straight wire actuator is supported by resiliently deformable wires rounded semicircular and attached to both ends thereof. By passing an electric current pulse through the straight wire actuator disposed in a vertical magnetic field, the straight wire actuator is shifted in the direction of the axis of the printing pin against the resiliency of the semicircular supporting wires to hit a platen roller through an ink ribbon so as to print an ink dot on a printing paper wrapped around the platen roller. After the ink dot printing, by applying a counter current pulse to the straight wire actuator, the printing pin is caused to return to its initial or rest position.
For effecting a high speed ink dot printing, the printing head is required to have an actuator operative at a high operating frequency. However, the conventional electrodynamic wire dot printing head is limited to at most 300 Hz in operating frequency and, therefore, unsuitable for high speed printing. The cause of the low operating frequency of the printing head is that, when the printing pin returns to its initial or rest position, the printing pin does not rest quickly and will rebound to oscillate diminishingly with respect to its rest position. If a current pulse is passed through the actuator wire during the diminishing oscillation, the magnetic force produced by the magnet is partially cancelled. As a result, it is hard to cause the printing pin to travel a sufficient stroke and to give an impact required to effect a desirable print of ink dot on the printing paper. For this reason, the conventional printing head is prevented from repeating to effect a successive printing until the printing pin completely rests in its rest position. Therefore, the period required to print one ink dot is prolonged, resulting in a slow speed dot printing.
There actually occurs a rebound of the printing pin, an ink dot will be overprinted double or triple and, thereby, a character or symbol formed by a lot of ink dots becomes non-uniform in depth. Moreover, if the printing paper is advanced during the double or triple overprinting, a distorted character will be printed.